Prior to this invention a wide variety of different molded corrugated plastic drainage pipe constructions have been purposed. Some examples of these prior constructions are described in the following U.S. patents: U.S. Pat. No. 3,753,352 (issued on Aug. 21, 1973), U.S. Pat. No. 3,861,153 (issued on Jan. 21, 1975), U.S. Pat. No. 3,885,799 (issued on Dec. 24, 1974), U.S. Pat. No. 3,861,152 (issued on Jan. 21, 1975), U.S. Pat. No. 3,832,855 (issued on Sept. 3, 1974), U.S. Pat. No. 3,802,202 (issued on Apr. 9, 1974), U.S. Pat. No. 3,747,352 (issued on July 24, 1973), U.S. Pat. No. 3,878,685 (issued on Apr. 22, 1975), U.S. Pat. No. 3,843,758 (issued on Oct. 22, 1974), U.S. Pat. No. 3,897,090 (issued on July 29, 1975), U.S. Pat. No. 3,929,359 (issued on Dec. 30, 1975), U.S. Pat. No. 3,926,222 (issued on Dec. 16, 1975), U.S. Pat. No. 3,789,615 (issued on Feb. 5, 1974 ), U.S. Pat. No. Des. 235,100 (issued on May 13, 1975), and U.S. Pat. No. Des. 235,103 (issued on May 13, 1975).
Thin-walled corrugated plastic drainage pipe of the type shown in these patents is particularly desirable for use in septic tank distribution systems on account of its lightweight, inexpensive construction. The lightweight, thin-walled construction is achievable because the corrugations add substantial strength to the pipe.
In the past, however, two problems have been encountered with corrugated plastic drainage pipe, particularly in the use of the pipe in buried septic tank distribution systems. One problem pertains to the clogging of the pipe's drainage holes to impair the drainage capacity of the pipe. The other problem involves the impairment of the pipe's structural strength and particularly to the pipe's crushing strength due to the location and size of the drainage holes.
In some corrugated drainage pipe constructions, such as the one described in U.S. Pat. No. 3,830,373, the drainage holes are located entirely in the roots or valleys of the corrugations. These drainage holes, according to U.S. Pat. No. 3,878,685, tend to clog because of their relatively small size. Enlarging the drainage holes to avoid clogging, however, is said in U.S. Pat. No. 3,878,685 to sever adjacent corrugations for the customary longitudinal spacing of the ribs or ridges of the corrugations. This weakens the strength of the pipe.
The solution offered by U.S. Pat. No. 3,878,685 to this problem is to provide the drainage holes in widened uncorrugated areas or flats. The uncorrugated flats are at the root level and are located at spaced apart intervals along the pipe between corrugated portions of the pipe. This solution, however, reduces the number of corrugations on the pipe. It additionally concentrates the drainage holes in confined regions of the pipe so that the holes are not uniformly distributed along the entire length of the pipe.
In another type of corrugated plastic drainage pipe construction (see U.S. Pat. No. 3,861,152) the ribs or ridges are circumferentially interrupted and the drainage holes are located in the interrupted regions. The interruptions of the ribs, however, are noted in U.S. Pat. No. 3,861,152 to weaken the pipe.
Placement of the drainage holes in the two foregoing patents permits liquid or effluent to flow circumferentially around the pipe in the valleys or root-level channels between ridges of the corrugations. This placement was discovered in the present invention to promote clogging of the bottom drainage holes (those holes facing vertically downwardly toward the pipe-supporting surface). In particular, it was found that effluent draining from the other sidewardly facing drainage holes is channeled to the bottom of the pipe and thus collects in the region of the bottom drainage holes. This collection of effluent forms iron oxide which tends to clog the bottom holes.
The iron oxide deposits result in the build-up of a black, gelatinous sludge in the valleys or root-level channels between ridges of the corrugations to promote clogging of the drainage holes.